My Webster's defines it as "to beat with a pistol".But where did such an odd phrase come from?
There's no similar expression with other weapons or objects . No such thing as to "baseball-bat whip", or to "brass-knuckle whip" , or "to blunt-instrument whip" , or "to rock whip" or "to 2 by 4 whip". But people can be beaten to death with all those things.
To me, saying 'whip' implies a device that is long and flexible (and, duh, whip-like, for lack of a better word) .Not a pistol.

Today's newspaper uses the phrase (in a story about the courtroom shooting in Atlanta) The criminal grabbed a policeman's gun, shot several people, then "pistol-whipped" a reporter and stole his car. Why not say he "assaulted and beat a reporter".?

Because the "preferred" method of using a baseball, brass knuckles, rock, 2x4, blunt instrument, etc., is to hit the person with it. Usually people shoot bullets at other people with guns. Hitting them with the pistol is a secondary use of the pistol.

Likewise you'd specify if someone launched a baseball bat or a rock out of a cannon at someone. Not the primary usage.

In the movies, and thus in real life, extremly stupid people are likely to hold the barrel in their hands, with the ...um....place where the bullet fires pointing towards themselves. "Never point a gun at anything you do not wish to destroy."

Not very smart. They then bring the handle down on the back of someones neck.

Also I suspect the idea of being beaten with a pistol is inherently more frightening than the idea of being beaten with, say, a small rock of the same mass. Because of the implied threat of escalation to using the pistol's business end.

It can also be pretty nasty by itself. You don't have to turn the gun around; instead, you use it as a weight to smash you hand into the victim.

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"East is East and West is West and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does."Purveyor of fine science fiction since 1982.

Yeah, I can see why getting beaten with a pistol, any part of it, would be pretty scary. Still, I gotta agree that the "whipped" part of the phrase has always confused me.

Uh, the mental picture I've always had is slightly more unpleasant: I've pictured a victim being bound and gagged, and having the perp use the gun sight on the pistol to cut up the victim's face or other body parts.

Is there any basis for this being part of the expression? Is it ever done? Often done? Ah, I'm probably just imagining things. Right?

There is a scene in the movie The Arrival, where Zane is confronted by a police officer. The officer approaches Zane with his gun pointed at him, and he keeps moving closer. Once he is "in his face", he pistol-whips him (still holding the gun as if to shoot) by sharply whipping the barrel of the gun at the lower cheek.

I always imagined that the person administering the pistol whipping holds the gun normally (i.e., as if to shoot), and brings down the butt end of the gun the victim. The weight of the gun adds force to the blow.

According to my copy of Kill or Get Killed by Colonel Rex Applegate, the proper method of pistol whipping is to hold the pistol by the grip in the normal fashion and strike with the barrel or slide. There are some pretty graphic line drawings of a German soldier getting his face caved in with a 1917 revolver and with a 1911A1.
Other sources indicate that in the Old West striking someone with a pistol was called "buffaloing" and that, once again, the barrel was the preferred part of the gun for impact.

It is hard to imagine how you could whip someone with an object such as a gun, however, www.dictionary.com offers the following description of 'whip': " To move in a sudden, quick manner; dart." So I guess in this case whipped simply refers to the speed of the motion rather than anything else.

Here is a picture of another hollywood depiction of pistol whipping, in this scene from 'Sneakers' the victim is lying in the boot of a car and the camera shot is from the victim's point of view. You can see the gun is being held like you would hold a rock or paperweight:

CHAPPACHULA
..There's no similar expression with other weapons or objects . No such thing as to "baseball-bat whip", or to "brass-knuckle whip" , or "to blunt-instrument whip" , or "to rock whip" or "to 2 by 4 whip". But people can be beaten to death with all those things..

From my recollection of Wyatt Earp's account of his life, he discussed pistol-whipping and fanning (where you use one hand to keep recocking the weapon.) His statement on pistol-whipping was that there is no point in holding the weapon any differently from how one regularly holds a gun. If you thwonk someone with a big hunk of metal it will hurt just fine--and holding correctly allows you to use the gun as intended after.
Fanning he said wasn't something anyone (with a brain) would ever do for real as it throws your aim off and--after practicing every day--you can recock your weapon quite fast enough regularly.